1. Field of the Invention
The present invention relates to a fluid bearing for supporting a rotary shaft by means of a pressurized fluid, and more particularly to a fluid bearing which is capable of preventing bearing seizure even if the rotary shaft continues to rotate due to its inertia after the electric supply is interrupted and, thus, supply of pressurized fluid is stopped.
2. Description of the Prior Art
In general, a bearing device for a spindle of a machine tool is subjected to an excessive load, such as a vibration load, during machining operation. A bearing device depending on only a static bearing support may not be able to bear such an excessive load. For this reason, there is provided in a hydrostatic pressure generating zone a land portion which is capable of heightening bearing rigidity for such a vibration load. In a normal condition, fluid is supplied to the land portion from a pressure fluid supply pump, however, if the pressure fluid supply pump is stopped due to interruption of electric supply and thus generation of hydrostatic pressure disappears, bearing seizure may be caused at the land portion where a radial load is applied due to a lack of lubricating fluid. In order to overcome this disadvantage, it has been considered to provide a fluid bearing which is capable of introducing fluid which is remaining on a bearing surface into a land portion. Such a fluid bearing is shown in FIGS. 1 and 2, wherein four pressure generating zone PZ as indicated in dotted lines are circumferentially arranged on the bearing surface of a bearing member 11. Each pressure generating zone PZ is provided with a pair of axially spaced fluid pockets 13a and 13b, a land portion 14 and a supply groove 13c. Each supply groove 13c is formed so as to connect the edge portions of the pair of fluid pockets 13a and 13b. The bearing member 11 is mounted on a grinding wheel support (not shown) in such a way that the supply groove 13c at a land portion 14 where a radial load F1 is applied is located in a lower position in a gravitational direction, so that fluid remaining on the bearing surface is collected in the supply groove 13c due to its gravity when the pressure fluid supply pump is stopped. Accordingly, even if a rotary shaft 12 continues to rotate due to its inertia immediately after the stoppage of the supply pump, the remaining fluid in the supply groove 13c is introduced into the land portion 14 together with the rotation of the rotary shaft 12, so that bearing seizure due to a lack of lubricating fluid may be prevented.
However, if a radial load F2 is applied to a land portion 14 opposite to the land portion 14 where the radial load F1 is applied, it may suffer from bearing seizure due to a lack of lubricating fluid, because a supply groove 13c at the land portion 14 where the radial load F2 is applied is located in a higher position in a gravitational direction and thus fluid is collected in the fluid pockets 13a and 13b instead of the supply groove 13c. Therefore, fluid is hardly introduced to the land portion 14.
Such a disadvantage occurs in the following occasions. Generally, the disposition of a grinding wheel relative to a pulley for driving the same in a straight type grinding machine is different from that of an angular type grinding machine. Thus, as viewed from a work table (not shown) on which a workpiece is mounted, bearing members used for a pulley side and for a grinding wheel side are disposed respectively on the right and left side in the straight type grinding machine and are conversely disposed respectively on the left and right side in the angular type grinding machine. It is impossible to adapt the bearing members used for the pulley side to that used for the grinding wheel side or vice versa, because there are many different parts. However, it is desirable to provide a bearing member used for the pulley side or the grinding wheel side which can be used in common for both straight type and angular type grinding machines.
When the bearing member used for the pulley side in the angular type grinding machine is used for the pulley side in the straight type grinding machine, it is disposed on the right side, as viewed from the work table, and a radial load which is caused by the weight of the rotary shaft 12 and the belt tention of the pulley is changed, for example, from F1 to F2 as shown in FIG. 2.
Accordingly, bearing seizure may be caused at the land portion 14 where the radial load F2 is applied, if a conventional bearing member such as shown in FIGS. 1 and 2 is used in common, because the supply groove 13c is located in a higher position.